In some computer applications, it is required to search in a video clip or in a photo for an object that matches a reference figure provided by a hand-drawn sketch. Most notably, an important one of these applications is in law enforcement where a picture of a criminal suspect is given as a hand-drawn sketch prepared by an artist according to description of an eyewitness.
In one typical approach, a computer search of an object that matches a hand-drawn sketch is carried out by first extracting statistical information of a feature (intensity, color, gradient, edge, texture, etc.) of the sketch and then identifying an object in the video clip or in the photo where the object has similar statistical information for the feature. For both a camera-captured photo/video and a hand-drawn sketch, the statistical information depends on the visual content that is encoded in the image. The photo/video truly reveals what is captured by the camera, whereas the sketch reveals the perceptual content of the artist or the author in general. It follows that the sketch somehow includes an element of a perceptual bias of the author. Taking into consideration the perceptual bias of the sketch in the search of a matched object is desirable in order to enhance the success rate of matching.
In WO2004/027692, a method for enhancing the success rate of sketch-photo matching is disclosed. The approach adopted in the method is to transform a photo image into a pseudo sketch, or an original hand-drawn sketch into a pseudo photo, such that the difference between the photo and the original sketch is reduced after transformation, thereby facilitating the computer search. Such transformation can be implemented in the presence of a database preferably containing all possible sketch types, a requirement that is difficult to fulfill. In the absence of such large database, a smaller database containing a sufficient number of photo-sketch pair examples may be used to facilitate training of the transformer on-the-fly. Nonetheless, an extra database is inevitably required in the implementation of the method disclosed in WO2004/027692.
In Content-Based Image Retrieval Using Hand-Drawn Sketches and Local Features: a Study on Visual Dissimilarity, a thesis authored by Folco Banfi and submitted to Université de Fribourg (Suisse), 2000, it is disclosed a weighting scheme having an effect similar to estimating perceptual bias levels. The weighting scheme gives pre-defined or pre-computed weights for sketches specifically drawn in certain types of sketch style or outline. Apart from the limitation that the weighting scheme is only for specific types of sketch style, the weights are fixed and are not obtained by taking into account individual variations from one hand-drawn sketch to another. Automatic computation of the weights based on a sketch to be used in the object search is more desirable.
There is a need in the art for automatic computation of a perceptual bias level evaluated for a hand-drawn sketch without a need for an extra database in the computation.